Then in any directory I can run "rake git:init" and it will do all the intial setup. The remote add is a little harder because the remote name would be a variable (could be provided via a shell variable or a Readline prompt).

For creating a rails app, I'd add an alias to ~/.bash_profile:

alias new_mysql="rails new $ARGV --database=mysql"

Then run "new_mysql myRailsProject".

For the most part I would think just running a bunch of command line scripts would be a shell alias rather than a Rake task.

You should be using rake tasks for this stuff. Bates has a screencast showing how to accomplish what your trying to get done. Basically you create a custom task and after you can call rake my_task and it will execute your script.